The present invention is related to a Wollaston prism formed by two birefringent wedges having optic axis to each other at right angle, to a Fourier-transform spectrometer comprising said Wollaston prism, and to a method to adjust said Fourier-transform spectrometer.
A Wollaston prism is comprised of two similar wedges of birefringent material joined by their hypotenuse to form a rectangular block. The optic axes within the two wedges are aligned perpendicular to each other and parallel to the entrance/exit faces of the composite block. The angle of refraction at the internal interface of the Wollaston prism depends on the polarization state of light and hence leads to the customary use of a Wollaston prism as a polarizing beam splitter.
Conventional Fourier-transform spectrometers are based on Michelson interferometers. When the output of the interferometer is recorded as a function of the path difference between two arms, an interferogram is obtained that is the autocorrelation of the optical field. The power spectrum of the Fourier transform of the interferogram corresponds to the spectral energy or power distribution of the input light. Draw-backs associated with these instruments are that high quality mirror-scanning mechanisms are required, and the temporal resolution is limited by the maximum mechanical scanning rate.
As an alternative, a Wollaston prism may be used in a Fourier-transform spectrometer with no moving parts. It is well known in the state of the art that when a Wollaston prism is placed between two suitably oriented polarizers and illuminated with a light source, a set of straight-line interference fringes will be produced localized to a plane within the prism. These fringes are the Fourier transform of the spectral power distribution.
Although the use of Wollaston prisms in Fourier-transform spectrometers has been proposed earlier, it could not be applied without a suitable scanning device having a high resolution. Such a high-resolution scanning device has been described by Takayuki Okamoto et. al. in "A Photodiode Array Fourier Transform Spectrometer based on a Birefringent Interferometer" (Applied Spectroscopy 40, p. 691 to 695, 1986).
The varying path difference across the Wollaston prism of the above-mentioned Fourier-transform spectrometer results in the formation of interference fringes localized to a imaging plane within the Wollaston prism. For that reason, an imaging lens must be provided to image the interference plane onto the scanning device in order to obtain best results with regard to the contrast of the image.
All of the above-described Fourier-transform spectrometer have the general draw-back that the angle of incidence for a light beam to be analyzed is very small, i.e. measures must be taken to reduce the angular extent of this light beam and the spectrometer must be precisely adjusted in order to obtain acceptable results.
To overcome this draw-back, a design of a static Fourier-transform spectrometer based on a Wollaston prism has been presented by J. Courtial et. al. in "Design of a static Fourier-transform spectrometer with increased field of view" (Applied Optics, Vol. 35, No. 34, Dec. 1, 1996, p. 6698-6702). The field of view is increased by including an achromatic .lambda./2-plate (.lambda.: wavelength) between the prisms or by combining prisms fabricated from positive and negative birefringent materials. These materials are expensive and are therefore not suitable for any mass product.